At present, in practice there exist only loudspeakers with conical diaphragms, the diameter of which is at least equal to 3 cm, and the depth of which always exceeds 2 cm. In general, conventional loudspeakers, especially those possessing some power, are of bigger dimensions, and given their customary shape and their bulk, they cannot be housed in a small volume and, in particular, in a narrow space. These drawbacks render current loudspeakers in particular unsuitable for use in certain specific applications, especially in active soundproofing systems which create "anti-noise".
However, .U.S Pat. No. 4,792,978 discloses a loudspeaker describable as "linear", in the sense that it possesses an elongate rectangular shape, and comprises a diaphragm of corresponding shape, set vibrating by an electromagnetic system with coil of oblong shape. The coil, attached to the diaphragm, here runs in a plane perpendicular to the said diaphragm. This coil moves between elongate permanent magnets of opposite polarities. It follows that the coil does not move in a homogeneous and closed magnetic field, thus detracting from the sound restitution quality of the coil.
U.S. Pat. No. 4,550,428 describes a loudspeaker of conventional general shape, that is to say circular, comprising a cylindrical coil parallel to the diaphragm, an annular permanent magnet and a central pole piece. Despite the arrangement of the coil, this document remains far from the subject matter of the present invention.
European patent application No. 0 710 946 reveals an acoustic attenuation device, having an active double wall, using loudspeakers arranged as the border of a double wall, such as a double glazing, in the air gap between the two walls. However, this device uses circular loudspeakers, of considerable diameter, and can therefore be applied only to double glazings with a thick air gap.
The present invention aims to remedy all the aforesaid drawbacks, by providing a loudspeaker of "linear" type, which can be housed in a reduced volume and especially in a narrow space, whilst possessing comparable efficiency to that of a conventional loudspeaker which would be unable to fit into the same volume, this loudspeaker moreover having excellent quality of sound restitution.
To this end, the subject of the invention is essentially a loudspeaker of elongate overall shape, termed a linear loudspeaker, comprising a parallelepipedal hollow body of elongate shape, a longitudinal face of which consists at least partially of a plane diaphragm of elongate rectangular shape, suspended by its edges from the adjacent lateral faces of the said body or under the base of the said body, and integral with an elongate internal rigid wing which runs in the longitudinal direction of the said body, substantially in the midplane of the latter, and which bears a moving electromagnetic coil of elongate profile, located between two fixed elongate magnets, associated with fixed pole parts, characterized in that the moving electromagnetic coil is parallel to the diaphragm, and arranged between two fixed elongate magnets located respectively along two longitudinal lateral faces of the said body, these lateral faces and the base of the body forming fixed pole parts, which also comprise a central fixed pole part, situated in the longitudinal midplane of the body and received in a longitudinal recess of the internal rigid wing of the diaphragm, the said diaphragm thus being stretched in such a way as to allow its symmetric deformation on either side of its position of rest, in the course of the vibrational motion of this diaphragm when the loudspeaker is excited.
According to a preferred embodiment of the invention, the elongate plane diaphragm is composed of a rigid part, covered with an elastic film, and/or comprises such a film, of which the edges are fixed onto the adjacent lateral faces or under the base of the body of the loudspeaker, this fixing being achievable by gluing or by mechanical crimping. The elastic film is itself attached to the rigid part of the diaphragm by gluing, or by overmolding, or else by a mechanical link.
Thus, the invention proposes a loudspeaker whose diaphragm is not conical as in a conventional loudspeaker, but possesses a specific geometry, in this instance a plane, rectangular and elongate shape, which is therefore narrow in one direction and highly stretched in another direction, in relation to the elongate parallelepipedal overall shape of the loudspeaker, thereby justifying its description as a linear loudspeaker. The diaphragm of the loudspeaker which is the subject of the invention possesses just one external or peripheral suspension, unlike the conical diaphragms of the conventional loudspeakers which possess an external suspension and an internal suspension (the latter being customarily described as a "spider"). Additionally, contrary to a conventional loudspeaker, not only certain pole parts but also the magnets are located outside the coil, the internal rigid leaf of the diaphragm being however recessed so as to receive the central fixed pole part, situated in the longitudinal midplane of the body of the loudspeaker.
The geometrical shape and the particular arrangement of the constituent elements of the loudspeaker thus achieve optimization, making it possible for example to obtain a loudspeaker with a width of 2 cm, a depth of 2 cm and a bigger or smaller length, for example of the order of 50 cm, possessing very satisfactory efficiency. In particular, given the considerable length of the diaphragm, the latter displaces a large mass of air during its vibration, thereby affording good efficiency in the low frequencies. By virtue of its elongate coil, running practically over the entire length of the diaphragm, beneath the latter, and to the compactness of the loudspeaker which is the subject of the invention.
Moreover, with the suspension proposed by the invention, the diaphragm is suspended in such a way as to be stretched permanently and in a "balanced" manner, that is to say engendering the same elastic restoring force for a displacement in either direction of the internal rigid part attached to this diaphragm, so that the latter has, in the course of its vibrational motion, a symmetric deformation on either side of its position of rest. Stated otherwise, the deformation of the vibrating diaphragm is the same (in absolute value), for a positive or negative half-wave of the electrical signal applied to the input of the loudspeaker. The symmetric deformation of the vibrating diaphragm makes it possible to ensure good linearity of electromechanical conversion, especially at low frequencies, by avoiding the distortion phenomena which disturb sound restitution in general but also for the particular applications of an active anti-noise system. The suspension embodied according to the invention is furthermore very flexible, this leading to high efficiency of the transducer.
It will also be noted that, by means of the tension of the diaphragm, the restoring force can be adjusted, as a function of the requirements of the passband needed for use, as well as the distortion needed for the application, be it soundproofing, or active anti-noise, by applying the law F=kx.
The orientation of the coil parallel to the diaphragm (whereas it is perpendicular thereto in the aforesaid U.S. Pat. No. 4,792,978), also leads to the coil being situated and displaced in a homogeneous and closed magnetic field, to which the central pole part contributes, this also being important in obtaining optimal behavior of the diaphragm.
According to one mode of execution of the invention, the diaphragm runs along the entire length of the body of the loudspeaker.